There are many kinds of implantable medical devices. Some monitor patient conditions while others disperse some form of therapy. One particular type of implantable medical device is an implantable cardiac therapy device, or ICTD. ICTDs are implanted within the body of a patient to monitor, regulate, and/or correct heart activity. ICTDs include implantable cardiac stimulation devices (e.g., implantable cardiac pacemakers, implantable defibrillators) that apply stimulation therapy to the heart as well as implantable cardiac monitors that monitor heart activity.
ICTDs typically include a control unit positioned within a casing that is implanted into the body and a set of leads that are positioned to impart stimulation and/or monitor cardiac activity. With improved processor and memory technologies, the control units have become increasingly more sophisticated, allowing them to monitor many types of conditions and apply tailored stimulation therapies in response to those conditions.
ICTDs are typically capable of being programmed remotely by an external programming device, often called a “programmer”. Today, individual ICTDs are equipped with telemetry circuits that communicate with the programmer. One type of programmer utilizes an electromagnetic wand that is placed near the implanted cardiac device to communicate with the implanted device. The wand contains a coil that forms a transformer coupling with the ICTD telemetry circuitry. The wand transmits low frequency signals by varying the current in a coil.
Early telemetry systems were passive, meaning that the communication was unidirectional from the programmer to the implanted device. Passive telemetry allowed a treating physician to download instructions to the implanted device following implantation. Due to power and size constraints, early commercial versions of the implanted devices were incapable of transmitting information back to the programmer.
As power capabilities improved, active telemetry became feasible, allowing synchronous bi-directional communication between the implanted device and the programmer. Active telemetry utilizes a half-duplex communication mode in which the programmer sends instructions in a predefined frame format and, following termination of this transmission, the implanted device returns data using the frame format. With active telemetry, the treating physician is able to not only program the implanted device, but also retrieve information from the implanted device to evaluate heart activity and device performance. The treating physician may periodically want to review device performance or heart activity data for predefined periods of time to ensure that the device is providing therapy in desired manner. Consequently, current generation implantable cardiac therapy devices incorporate memories, and the processors periodically sample and record various performance parameter measurements in the memories.
Current telemetry systems have a limited communication range between the programmer wand and the ICTD, which is often referred to as “short-range telemetry” or “wand telemetry”. For effective communication, the wand is held within two feet of the ICTD, and typically within several inches. One problem is that the ICTD has insufficient power to transmit longer range signals. Another consideration is the inherent EMI-resistant design of the ICTD. The ICTD circuitry is typically housed in a hermetically shielded can to prevent electromagnetic interference (EMI) from disrupting operation. The can prevents penetration of high frequencies, thereby limiting communication to the low frequency range of less than 200 KHz. In one exemplary system, signals sent from the programmer to the implanted device are transmitted on a carrier of approximately 36 KHz, and data is transmitted to and from the implanted device at approximately 8 KBaud.
Conventionally, data about a patient's cardiac condition is gathered and stored by the programmer during programming sessions of the ICTDs. Analysis of the cardiac condition is performed locally by the programming software. Programmers offer comprehensive diagnostic capabilities, high-speed processing, and easy operation, thereby facilitating efficient programming and timely patient follow-up.
In addition to local analysis, TransTelephonic Monitoring (TTM) systems are employed to gather current cardiac data of patients when the patient is remote from the healthcare provider. TTM systems are placed in patients' homes. They typically include a base unit that gathers information from the ICTD much like the programmer would. The base unit is connected to a telephone line so that data may be transmitted to the medical staff responsible for that patient. An example of an ICTD TTM system is a service from St. Jude Medical® and Raytel® Cardiac Services called “Housecall™.” This service provides current programmed parameters and episode diagnostic information for a plurality of events including stored electrograms (EGMs). Real-time EGMs with annotated status information can also be transmitted.
Using a telephone and a transmitter, the TTM system provides both the medical staff and the patient the convenience of instant analysis of therapy without having the patient leave the comfort of home. Typically, real-time measurements are transmitted in just minutes. Patients may be closely monitored, and the medical staff has more control of their patient's treatment, thus administering better patient management.
While strides have been made for improving patient monitoring, there remains an ongoing need to improve the communication capabilities between implanted devices and external devices, particularly the need to communicate more effectively over greater transmissions ranges.